Buy a first-class ticket. Use it to get into the VIP lounge at the airport. Then change the itinerary. Repeat.
Yes, somebody did this more than 300 times with a ticket on China Eastern Airlines. Then s/he returned the ticket for a full refund.
Story from Kwong Wah e-news.
Dunno about China Eastern, but the US domestic tickets I’ve bought recently have steep charges for making changes. Recently the cost of changing one leg of a $225 round-trip ticket was $150.
You can buy first-class and business-class tickets with no restrictions, fully refundable, on many (maybe all) airlines.